In exhaust gas purification systems equipped with a PM trapper that traps particulate matter (which will be hereinafter abbreviated as “PM”) in the exhaust gas from an internal combustion engine, regeneration process for removing the trapped or collected PM from the PM trapper by oxidizing it is performed at a time when a certain amount of PM has been collected on the PM trapper.
If the regeneration process is performed for a PM trapper that is in an “over-collection” state for some reason, oxidation reaction of a large amount of PM may occur drastically, and a fault such as melting of the PM trapper may be caused by the reaction heat generated therein in some cases.
Heretofore, technologies intended for early detection of such a fault have been proposed. For example, Japanese Patent Application Laid-Open No. 6-323127 discloses a technology of detecting a fault in a PM trapper based on a comparison of a differential pressure across the PM trapper just after execution of regeneration process and the minimum value of the differential pressure across the PM trapper at the time when it was brand-new.
Furthermore, Japanese Patent Application Laid-Open No. 2004-308454 discloses a technology intended to detect a fault in a PM trapper accurately at an early time after the occurrence of the fault by measuring the pressure of the exhaust gas downstream of the PM trapper, calculating the amplitude of pulsation of the pressure, and making a determination that a fault is present when the amplitude of pulsation exceeds a specific value.
Still further, Japanese Patent Application Laid-Open No. 2003-155920 discloses an abnormality detection apparatus for PM trapper that is provided with a differential pressure sensor that detects a differential pressure across a PM trapper and a determination device that makes a determination that the PM trapper is clogged when an abnormal increase in the back pressure is detected based on the differential pressure actually measured by the differential pressure sensor and makes a determination that the PM trapper is damaged by melting when an abnormal decrease in the back pressure is detected, wherein the determination device is configured to subtract, upon making a determination, a value corresponding to an increase in the backpressure caused by ash that has deposited over time from the actually measured differential pressure to enable reliable determination of occurrence of clogging and melting damage in the PM trapper.
Japanese Patent Application Laid-Open No. 2004-353606 discloses a technology concerning an internal combustion engine having a first stage catalyst provided in an exhaust passage and having an oxidizing ability, a PM trapper provided downstream of the first stage catalyst and reducing agent supply means for supplying reducing agent from upstream of the first stage catalyst, in which the reducing agent is supplied by the reducing agent supply means to remove PM trapped by the PM trapper thereby regenerating the PM trapper, wherein a first exhaust temperature sensor that senses the temperature of the exhaust gas downstream of the first stage catalyst and upstream of the PM trapper and a second exhaust gas temperature sensor that senses the temperature of the exhaust gas downstream of the PM trapper are provided, and the larger the difference between the exhaust gas temperature sensed by the first exhaust gas temperature sensor and the exhaust gas temperature detected by the second exhaust gas temperature sensor after a lapse of specified time since the start of removal of PM by the reducing agent supply means is, the higher the degree of deterioration of the first stage catalyst is assessed to be.
Furthermore, Japanese Patent Application Laid-Open No. 2005-201119 discloses a technology for diagnosing clogging of an NOx catalyst, in which there are provided an NOx catalyst disposed in an exhaust passage, a DPNR catalyst disposed downstream of the NOx catalyst, a first temperature sensor that senses the temperature of the exhaust gas that has just passed through the NOx catalyst and a second temperature sensor that senses the temperature of the exhaust gas that has just passed through the DPNR catalyst, and clogging of the NOx catalyst is diagnosed based on the difference between the exhaust gas temperature sensed by the first temperature sensor and the exhaust gas temperature sensed by the second temperature sensor.
Japanese Patent Application Laid-Open No. 2004-132358 discloses a technology concerning a control apparatus that estimates the amount of PM deposited in a PM trapper and performs a regeneration process for the PM trapper based on the estimation result, in which there are provided operation state detection means for detecting the operation state of an internal combustion engine, first estimation means for estimating the amount of PM deposited in the PM trapper based on a differential exhaust gas pressure across the PM trapper, second estimation means for estimating the amount of PM deposited in the PM trapper based on the operation state of the internal combustion engine and difference calculation means for calculating the difference between an estimated value of the deposited PM amount obtained by the first estimation means and an estimated value of the deposited PM amount obtained by the second estimation means. An estimated value obtained by either the first estimation means or the second estimation means is selected in response to the difference calculation means and the operation state detection means, and timing of performing regeneration process for the PM trapper is determined according to the selected estimated value so as to perform the regeneration process for the PM trapper at appropriate timing.